(15 pts) In the study described in problem 1, the investigators summarized the data in the following 2x2 table based on whether the CD4 count dropped below 200 by the end of the study. The following table shows their observed data:

	CD4<200atendofstudy	CD4>=200atendofstudy	Total
ARV1	50	190	240
ARV2	90	170	260
Total	140	360	500

1. Whatisthe ARV1-to-ARV2 odds ratioforthisdata? What is the ARV1-to-ARV2 relative risk (also known as the Risk Ratio)?

2. If you don't know how the sampling was designed (based on ARV, based on CD4 count, or based on taking a group without regard to ARV or CD4 count), is it better to use the odds ratio or the relative risk? Why (briefly)?

3. Whatisthenullhypothesis,expressedintermsofwhichever measure you chose in part b?

4. Fill in the following table with the expected counts, under the null hypothesis that there is no association between ARV regimen and probability of CD4<200 by the end of the study

ARV1

ARV2

Total

5. Using your answers to part (d), compute the chi-square statistic.